The flow field around lifting surfaces, such as aircraft wings and helicopter rotor blades, determines the pressure distribution over these surfaces and, therefore, determines the resulting aerodynamic forces and moments acting upon such surfaces. The modification and control of those forces can thus be achieved through modification and control of the flow field.
A number of methods of controlling the fluid flow around wings and rotors have been proposed and implemented. In the field of fixed wing aircraft, the most common method for controlling the flow around and consequently the forces generated by the wing is the use of a movable trailing edge surface, usually in the form of a flap. Basically, moving the trailing edge surface downward increases both the camber and the angle of attack of the wing thus modifying the flow field around the wing and causing the negative pressure distribution along the upper surface of the wing to increase in intensity. As a result, the lift force imparted to the aircraft is increased.
This solution, however, has a number of known disadvantages. Firstly, the trailing edge surface needs to be actuated, usually with a hydraulic actuator or actuators, which results in weight increase and added mechanical complexity. Secondly, when rapid movements of the trailing edge surface are required, limitations arise due to inertia and power/force limits of the actuating system. While not being particularly prohibitive in the case of fixed wing designs, these issues virtually eliminate the option of using flaps or other trailing edge surfaces on a moving wing such as the rotor blades of a helicopter. This is because such a rotor blade typically makes several rotations per second and a mechanically actuated flap system cannot respond rapidly enough.
Alternate methods for controlling the fluid flow around aerodynamic surfaces have been investigated. The blowing of fluid jets tangentially from one or more slots located on the surface of a wing to control the flow patterns about the wing has been established as a viable solution. The method is generally known as airfoil circulation control through blowing. Depending upon its characteristics, tangential blowing can be used to achieve two goals. The first goal is to energize the flow in the vicinity of the surface, known as the boundary layer and, therefore, delay the onset of flow separation and its adverse stalling effects. The second goal is an increase in the lift generated by the wing through an increase in the bound circulation of the airfoil. In this second case, blowing is performed over a trailing edge modified from a sharp point to a smoothly curved Coanda surface at the trailing edge. The blown jets and the adjacent flow will follow the Coanda surface until the balance between the pressure variation normal to the surface and the centrifugal force exerted on the jet is lost. The effect is to cause a change in the position of the stagnation points, a modification of the entire flow pattern, and a corresponding modification of the pressure distribution along the surface of the wing. The changes in pressure distribution and, consequently, the changes in aerodynamic forces and moments can be comparable to those created by a mechanical flap. Blowing thus has potential as an alternative to moving trailing edge surfaces such as flaps, particularly when the later solution is not practical, such as in controlling the lifting properties of helicopter rotor blades.
Both steady and unsteady blowing have been investigated, and certain benefits of unsteady blowing have been identified. The term "unsteady blowing" may include, for example, varying the flow rate of the blowing with time or, alternatively, changing the direction of or "vectoring" the blown jet as a function of time. A combination of variable flow rate and vectoring is also possible and useful. With regard to such unsteady blowing, one challenge for those skilled in the art has been developing effective methods of generating and controlling the characteristics of the blown jet rapidly enough to respond to real time flight correction or to control the lift characteristics of a helicopter rotor blade as it spins.
A recent attempt to control the blowing of compressed air from a wing is represented in the disclosure of U.S. Pat. No. 4,626,171 of Carter, Sr. et al. The method taught by Carter employs a chamber filled with pressurized air, which is ejected through a slot near the trailing edge of the wing. The air is expelled from the slot and travels along the wing surface and around the Coanda surface at the wings trailing edge. Thus, the flow pattern about the wing is modified as discussed above. To control the blowing, Carter discloses the use of large screws to adjust the maximum opening size of the blowing slot and thus control the rate at which compressed air is expelled from the slot. The pressure of compressed air inside the chamber controls the deflection of a portion of the slot opening thereby controlling the range of slot opening sizes up to the maximum set by the large screws. The pressure in the chamber directly controls the size of the slot opening. Thus, the rate at which the compressed air is expelled is increased by increasing pressure in the chamber and decreased by decreasing pressure in the chamber.
An initial difficulty with the system disclosed in the Carter patent is that the response time of the system is large. If one wishes to modify the normal flow about the wing, one must increase the rate at which compressed air is expelled from the slot by increasing the size of the slot opening. This in turn requires an increase in the pressure inside the chamber and substantial elapsed time for the pressure to build to a sufficient level to bend the portion of the wing forming the movable part of the blowing slot. The response time of a control input to decrease the slot size by decreasing the pressure in the chamber may be even longer. Reducing the slot opening size requires bleeding pressure from the chamber and this process can take up to several seconds to complete. Such slow response times and the inherent inaccuracies in measuring and regulating chamber pressure prohibit use of the system described in Carter for helicopter rotor blades or other applications requiring rapid cyclical blowing to modify airflow patterns at rapid rates.
Other attempts to control blowing from aircraft wings to control lift have lead to mechanical control systems. U.S. Pat. No. 4,966,526 of Amellio, et al. Discloses such a mechanical system for controlling the slot size and thus the rate of blowing from the slot. The Amellio patent illustrates several embodiments using a camming system to alter the size of the slot opening mechanically. This mechanical system, however, is bulky and as such can not be used in many applications. Additionally, the weight penalties for this system are substantial. Just as with most mechanical systems, maintenance and installment costs are also typically quite high. Finally, response time is not dramatically improved over the Carter patent discussed earlier. Not only would such response times be prohibitively slow for use of this system with a helicopter rotor blade, the extreme mechanical complexity required at the rotor hub to cycle the control system at rotor rates would be prohibitive.
In addition to all the inadequacies with the prior art discussed above, there are other more general shortcomings of the various systems for controlling blowing disclosed in the prior art. Most such systems depend on a thin spanwise slot, i.e. a slot that extends substantially the entire length of the wing, to deliver the air stream out of the wing surface. As such, there is no effective means for controlling the airflow independently at different points along the slot or along the wing. In other words, the air flow out of the slot cannot be varied as a function of position along with wing span. Any attempt to do so would greatly increase the mechanical complexity of the system.
Since mechanical flaps are not practical for rotor blades, helicopters and other rotorcraft historically have used a swashplate system located at the rotor hub. Through such a system, the rotor blades of the helicopter are cyclically pitched as they travel around the rotor hub. The result is a changing of each rotor blade's relative angle of attack as a function of its rotary position. This, in turn, changes the airflow and lift characteristics of the blades as they travel around the rotor hub. While swashplate systems have been successful and are used in virtually all commercial and military helicopter designs, they are nevertheless plagued with inherent problems primarily because the mechanical complexity of a swashplate system renders it difficult and expensive to maintain. In addition, the high cycle rates required induces substantial mechanical vibration and noise in the aircraft and stresses the mechanical components of the entire drive system.
Thus, there exists a specific need for a method and apparatus for controlling the blowing of air over lifting surfaces that overcomes the problems of the prior art by providing a system with reduced mechanical complexity, high reliability, and fast response time. Further, while rapid, light weight, mechanically simplified, and highly reliable blowing control is particularly useful for use in aircraft lift surfaces, such control has many applications outside the aircraft industry. For example, a vectored blowing system might be useful in submarines or spray painting devices. Creation of efficient clean burning gas flames with controllable directivity might also be a beneficial use of such a system. Thus, a general need exists for a light weight blowing control system, and particularly a vectored blowing control system, for use in a variety of applications. It is to the provision of such a method, apparatus, and system that the present invention is primarily directed.